Photodiode

ABSTRACT

A photodiode has an optical absorption layer composed of a depleted first semiconductor optical absorption layer with a layer width W D  and a p-type neutral second semiconductor optical absorption layer with a layer width W A . The ratio between W A  and W D  is set such that the total carrier transit time τ tot  becomes minimum in the optical absorption layer. The photodiode can further include a depleted semiconductor optical transmission layer with a bandgap greater than that of the first semiconductor optical absorption layer, between the first semiconductor optical absorption layer and an n-type semiconductor electrode layer.

TECHNICAL FIELD

The present invention relates to a photodiode, and more particularly toa broadband, high internal quantum efficiency, ultrafast photodiode thatcan greatly alleviate the problem of “tradeoff between response speedand internal quantum efficiency”.

BACKGROUND ART

Conventional photodiodes used as a photodetector are roughly dividedinto a pin-photodiode (pin-PD) and a unitraveling-carrier photodiode(UTC-PD) according to their structures.

The pin-PD has a structure in which an intrinsic (i-type) opticalabsorption (=active) layer that is depleted in a reverse-biased state issandwiched by a large band gap p-type electrode layer and n-typeelectrode layer. For a required frequency range of response, thethickness of the active region is designed and internal quantumefficiency is determined.

On the other hand, the UTC-PD has a structure in which a p-type neutraloptical absorption layer that is doped beyond a predeterminedconcentration to prevent the depletion in the reverse-biased state and alarge band-gap i layer that is depleted in the reverse-biased state aresandwiched by a p-type electrode layer and n-type electrode layer. Theoperation principle of the photodiode is described in Japanese patentapplication laid-open No. 9-275224 (1997) in more detail.

In addition, to solve a problem of a conventional photodiode in that itsoptical absorption layer must be thickened to improve the photoelectricconversion efficiency, and hence it cannot respond to a high-speedoptical signal, Japanese patent application laid-open No. 10-233524(1998) discloses a hybrid semiconductor photo-detector. The hybridsemiconductor photo-detector has the optical absorption layer composedof two layers, a p-type upper optical absorption layer and a highresistance n-type lower optical absorption layer, there by implementingthe pin-PD structure and UTC-PD structure with a substantially singlestructure.

The semiconductor photo-detector with such a structure can achievestable response speed with little variations. This is because when aspecified reverse-bias is applied across the two optical absorptionlayers, the high resistance n-type lower optical absorption layer isdepleted in its entirety to increase the drift speed of the photoexcitedholes, and the p-type upper optical absorption layer provides highdiffusion speed to minority electrons, which contributes to thephotoelectric conversion, even if the entire p-type layer is notdepleted.

However, Japanese patent application laid-open No. 10-233524 (1998)places main emphasis upon increasing the efficiency of the photodiode,and does not disclose how to design the structure of the diode for afrequency response bandwidth required. From the beginning, it has notbeen discussed in this technical field as to whether the structure ofthe diode with the two optical absorption layers is advantageous or notto increase the speed of the photodiode.

When light is launched onto the photodiode, the incident light generateselectron-hole pairs in the optical absorption layer. These electrons andholes are separated in the layer, causing a current to flow through anexternal electronic circuit. Generally, as the optical absorption layerbecomes thicker, the response speed of the photodiode is reduced becauseof the prolonged carrier transit time through the layer, but the activeregion can absorb the light more sufficiently, thereby improving theinternal quantum efficiency. In other words, there is a tradeoff betweenthe response speed and the internal quantum efficiency the mostimportant two factors determining the performance of the photodiode, viathe thickness of the optical absorption layer, and the compromisebetween them is primarily important.

The intrinsic response speed determined by the carrier transit speedwill be described briefly. The response speed of the pin-PD is almostdetermined by the transit time of the holes with lower drift speed. Thetransit time τ_(D) of the holes approximated by neglecting the transitspeed of the electrons is given by the following expression underuniform optical illumination,τ_(D)(pin)=W _(D)/3v _(h).  (1)The frequency response (3-dB down bandwidth: f_(3dB)), which is ameasure of response speed, is approximated by the following expression,f _(3dB)(pin)=1/(2πτ_(D)),  (2)where v_(h) is the drift speed of the holes, and W_(D) is a depletionlayer width.

On the other hand, in the UTC-PD, the electron transit speed in thei-layer with the large band gap depleted in the reverse-biased state ismuch greater than the electron transit speed in the p-type neutraloptical absorption layer. Accordingly, an effective carrier transit timeτ_(A) is substantially controlled by the optical absorption layer withthe slower electron transit speed, and is given by the followingexpression under the uniform optical illumination,τ_(A)(UTC−PD)=W _(A) ²/3D _(e) +W _(A) /v _(th),  (3)assuming diffusive electron transport.In addition, the frequency response (3-dB down bandwidth: f_(3dB)) isalso determined by the diffusion current of the electrons, and isapproximated by the following expression,f _(3dB)(UTC−PD)=1/(2πτ_(A)),  (4)where D_(e) is a diffusion coefficient of the electrons, v_(th) is athermionic emission velocity of the electrons, and W_(A) is the width ofthe p-type neutral absorption layer.

According to expressions (1)-(4), the dependence of the 3 dB bandwidthon the width of the optical absorption layer is given by the followingexpression for the pin-PD,f_(3dB)(pin)∝1/W_(D).  (5)

As for the UTC-PD, it is given by the following expression when the termW_(A)/v_(th) is relatively small,f_(3dB)(UTC−PD)∝1/W_(A) ².  (6)Thus, the dependence of the 3 dB bandwidth on the width of the opticalabsorption layer differs greatly for the pin-PD and UTC-PD.Specifically, it shows an inclination that the bandwidth of the pin-PDis high in a region where the optical absorption layer is thick, and thebandwidth of the UTC-PD becomes high as the optical absorption layerbecomes thinner.

To design the high-speed photodiode with increased response speed, it isadvantageous to employ the UTC-PD structure. In this case, however, theoptical absorption layer must be made thinner, which brings about thereduction in the internal quantum efficiency. Consequently, although theUTC-PD can achieve high speed operation, it leaves the problem of the“tradeoff between the response speed and internal quantum efficiency”,which reduces the internal quantum efficiency of the device for thehigh-speed operation.

DISCLOSURE OF THE INVENTION

The present invention is implemented to solve the foregoing problems.Therefore it is an object of the present invention to provide a broadbandwidth, high internal quantum efficiency photodiode giving thesolution to the problem of the “tradeoff between the response speed andinternal quantum efficiency”.

To accomplish the object of the present invention, there is provided aphotodiode including an n-type semiconductor electrode layer, an opticalabsorption layer having a first semiconductor optical absorption layerand a second semiconductor optical absorption layer having a p-type, anda p-type semiconductor electrode layer, which are stacked sequentially,the photodiode further including an n-type electrode formed on then-type semiconductor electrode layer, and a p-type electrode formed onthe p-type semiconductor electrode layer, wherein when a specifiedreverse-bias is applied across the n-type electrode and the p-typeelectrode, the first semiconductor optical absorption layer is depleted,and the second semiconductor optical absorption layer remains neutral ina region other than a region near an interface with the firstsemiconductor optical absorption layer, and wherein in the opticalabsorption layer with a fixed layer width equal to a sum (W=W_(D)+W_(A))of a layer width (W_(D)) of the first semiconductor optical absorptionlayer and a layer width (W_(A)) of the second semiconductor opticalabsorption layer, a ratio between W_(D) and W_(A) is determined suchthat |Δτ_(A)|=|−Δτ_(D)| holds, where −Δτ_(D) is a decrease of a holetransit time through the first semiconductor optical absorption layerand Δτ_(A) is an increase of an electron transit time through the secondsemiconductor optical absorption layer, when the layer width of thefirst semiconductor optical absorption layer is made W_(D)−ΔW, and thelayer width of the second semiconductor optical absorption layer is madeW_(A)+ΔW.

In the photodiode, the first semiconductor optical absorption layer andthe second semiconductor optical absorption layer may be composed of thesame or different compound semiconductor with composition represented byIn_(x)Ga_(1-x)As_(y)P_(1-y), where 0≦x and y≦1. In addition, the ratiobetween the W_(D) and W_(A) may be set such that τ_(tot) given by thefollowing expression becomes minimum,

 τ_(tot) =W _(A) ³/(3WD _(e))+W _(A) ²/(Wv _(th))+W _(D) ²/(3Wv _(h)),

where D_(e) is a diffusion coefficient of electrons, v_(th) is athermionic emission velocity of the electrons, and v_(h) is a driftspeed of holes.

Moreover, the photodiode may further comprise a semiconductor opticaltransmission layer between the first semiconductor optical absorptionlayer and the n-type semiconductor electrode layer, the semiconductoroptical transmission layer having a band gap greater than the firstsemiconductor optical absorption layer, wherein the semiconductoroptical transmission layer is depleted when a specified reverse-bias isapplied across the n-type electrode and the p-type electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a structure of a photodiode inaccordance with the present invention;

FIG. 2 is a diagram illustrating the carrier transit time in thesemiconductor optical absorption layer constituting the photodiode inaccordance with the present invention as shown in FIG. 1;

FIG. 3 is a band diagram illustrating the carrier drift in thesemiconductor optical absorption layers constituting the photodiode inaccordance with the present invention;

FIG. 4 is a graph illustrating the calculation results of the totalcarrier transit time τ_(tot) in the optical absorption layer and the 3dB bandwidth f_(3dB) of the photodiode in accordance with the presentinvention including a depleted InGaAs optical absorption layer with alayer width W_(D) and a p-type neutral InGaAs optical absorption layerwith a layer width W_(A);

FIG. 5 is a cross-sectional view showing another structure of thephotodiode in accordance with the present invention; and

FIG. 6 is a band diagram illustrating the carrier drift in thesemiconductor optical absorption layer constituting the photodiode inaccordance with the present invention as shown in FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described withreference to the accompanying drawings. A design idea of the photo diodein accordance with the present invention is as follows. Generally, whenoptical absorption occurs in halved regions I and II, the opticalfrequency response in the I region includes the structure parameters ofthe II region, and the optical frequency response in the opticalabsorption region of the II region includes the structure parameters ofthe I region. In other words, the response delay time of the I region isa function of the carrier transit time of the II region as well as thatof the I region. As for the response delay time of the II region, asimilar relationship holds. Accordingly, the total response of thestructure, which consists of the I region and II region coupled, differsfrom the response of the parallel superposition of the responses of thestructures consisting of the I region and II region independently. Forexample, doubling the layer width of the optical absorption layer in thepin-PD doubles the delay time and halves the bandwidth.

If it is possible to reduce the “mutually dependent relationship betweenthe I region and II region”, the total response can be obtained bysuperimposing the responses of the structure in parallel which includesthe I region and II region independently.

(EMBODIMENT 1)

FIG. 1 is a cross-sectional view showing a structure of the photodiodein accordance with the present invention. The photodiode includes ann-type semiconductor electrode layer 11 composed of a single-layer ormultiple-layers with different band gaps; a first semiconductor opticalabsorption layer 12 with a film thickness W_(D), which is depleted inthe reverse-biased state; a second semiconductor optical absorptionlayer 13 with a film thickness W_(A) which is p-type neutral in thereverse-biased state; and a p-type semiconductor electrode layer 14composed of the single-layer or multiple-layers with different bandgaps. The photodiode is composed of these layers stacked sequentially,and an n-type electrode 15 formed on the n-type semiconductor electrodelayer 11, and a p-type electrode 16 formed on the p-type semiconductorelectrode layer 14. The ratio between the width of the firstsemiconductor optical absorption layer 12 and that of the secondsemiconductor optical absorption layer 13 is determined such that theresponse speed becomes maximum in the specified operation conditionsunder the condition that the sum W of the two layer widths(W=W_(D)+W_(A)) is kept constant.

As long as W is fixed, in spite of variations in the ratio between W_(A)and W_(D), the total optical absorption coefficient of the semiconductoroptical absorption layer little varies, which is composed of the firstsemiconductor optical absorption layer and the second semiconductoroptical absorption layer. Accordingly, the internal quantum efficiencyof the photodiode is nearly constant, and hence the performance of thephotodiode is just concerned with the response speed.

FIG. 2 is a schematic diagram illustrating the carrier transit time inthe semiconductor optical absorption layer as shown in FIG. 1. Itschematically illustrates the carrier transit time (τ_(D), τ_(A)) forthe individual semiconductor optical absorption layers, the sum of thecarrier transit times, and the total carrier transit time (τ_(tot)) ofthe effective semiconductor optical absorption layer, while varying theratio between the layer width of the depleted first semiconductoroptical absorption layer 12 with the layer width W_(D) and that of thep-type neutral second semiconductor optical absorption layer 13 with thelayer width W_(A) under the condition of maintaining the sum of thelayer widths W (=W_(D)+W_(A)) at a fixed value.

As seen from FIG. 2, the total carrier transit time τ_(tot) of theoptical absorption layer, which is the factor that determines theresponse speed of the photodiode, differs from the simple sum of theelectron transit time τ_(A) in the W_(A) region and the hole transittime τ_(D) of the W_(D) region. This is because a current caused bycarriers generated in the depleted semiconductor optical absorptionlayer 12 and a current caused by carriers generated in the p-typeneutral semiconductor optical absorption layer 13 flow through theoptical absorption layers at the same time.

FIG. 3 is a band diagram illustrating the drift of the carriersgenerated in the p-type neutral semiconductor optical absorption layerand the depleted semiconductor optical absorption layer. When thesemiconductor optical absorption layers are irradiated of thesemiconductor structure including an n-type semiconductor electrodelayer 31 composed of a single-layer or multiple-layers with differentband gaps, a first semiconductor optical absorption layer 32 with a filmthickness W_(D) which is depleted in the reverse-biased state, a secondsemiconductor optical absorption layer 33 with a film thickness W_(A)which is p-type neutral, and a p-type semiconductor electrode layer 34composed of the single-layer or multiple-layers with different bandgaps, which are stacked sequentially, electron-hole pairs are generatedin the first semiconductor optical absorption layer 32 and secondsemiconductor optical absorption layer 33. The electrons drift towardthe n-type semiconductor electrode 31, whereas the holes drift towardthe p-type semiconductor electrode 34, thereby implementing thecondition in which the current flows through a parallel circuit, so tospeak.

In this case, the dependence of τ_(A) on W_(A) and the dependence ofτ_(D) on W_(D) are τ_(A)∝W_(A) ², and τ_(D)∝W_(D), respectively, whichmeans that their dependence on the optical absorption layer widthdiffers. Thus, if the relationship that the increase in τ_(A) (Δτ_(A))is less than a decrease in τ_(D)(−Δτ_(D)) is satisfied, that is, if|Δτ_(A)|<|−Δτ_(D)|, in the condition that the optical absorption layerwidth W is constant, the total carrier transit time τ_(tot) of theoptical absorption layer can be reduced by replacing part of the entireoptical absorption layer with the layer width of W by the p-type neutraloptical absorption layer with the layer width of W_(A). Consequently,the effective carrier transit time of the diode in its entirety becomesminimum when |Δτ_(A)|=|−Δτ_(D)|. The phenomenon can be understood byhandling the carrier generating behavior in the optical absorption layerby a simple charge control model, while carrying out uniform opticalirradiation with maintaining a carrier generating rate G(cm⁻³·s⁻¹) perunit volume at a constant value.

The optical illumination forms electron-hole pairs in the opticalabsorption layer, thereby generating a photocurrent J (=qGW). Generally,as the photocurrent increases, an electron charge −Q_(A) in the p-typeneutral semiconductor optical absorption layer and a hole charge Q_(D)in the depleted semiconductor optical absorption layer and itsneighborhood also increase.

The electron charge −Q_(A) in the p-type neutral semiconductor opticalabsorption layer is given by the following expression in a diffusionmodel,−Q _(A) =−qG[W _(A) ³/(3D _(e))+W _(A) ² /v _(th)].  (7)

Considering that τ_(A) is given byτ_(A) =ΔQ _(A) /ΔJ _(A),  (8)where ΔQ_(A) is a differential amount of Q_(A), and ΔJ_(A) is adifferential amount of the photocurrent J_(A) (=qGW_(A)) flowing throughW_(A) layer, τ_(A) is given by the following expression,τ_(A) =[W _(A) ²/(3D _(e))+W _(A) /v _(th)].  (9)

On the other hand, Q_(D) is given by the following expression because itincreases by an amount of qGW_(D) ²/2V_(h) in the depleted opticalabsorption layer, and decreases by an amount of qGW_(D) ²/3V_(h) at theinterface between the p-type neutral optical absorption layer and thedepleted optical absorption layer,

 Q _(D) =qGW _(D) ²/6v _(h).  (10)

Considering that τ_(D) is given byτ_(D)=2ΔQ _(D) /ΔJ _(D),  (11)where ΔQ_(D) is the differential amount of Q_(D), and ΔJ_(D) is thedifferential amount of the photocurrent J_(D) (=qGW_(D)) flowing throughthe W_(D) layer, τ_(D) is given by the following expression,τ_(D) =W _(D)/3v _(h).  (12)The coefficient two must be present (attached) to the right-hand side ofexpression (11) because the hole current is half the total current, andτ_(D) is treated here taking only hole transport.

Next, the optical frequency response of the photodiode with thestructure as shown in FIGS. 1 and 3 will be described in general. Assumethat R₁(ω) is an optical frequency response when an optical signal isinput to the first semiconductor optical absorption layer (depletionlayer), and that R₂(ω) is an optical frequency response when an opticalsignal is input to the second semiconductor optical absorption layer(p-type neutral layer). The electrons generated in the secondsemiconductor optical absorption layer are injected into the firstsemiconductor optical absorption layer and pass through it. Thus, R₂(ω)is given by a product R₂₂(ω)·R₂₁(ω), where R₂₂(ω) is an independentresponse of the second semiconductor optical absorption layer, andR₂₁(ω) is a response involving the passing of the electrons through thefirst semiconductor optical absorption layer. Since the presentphotodiode is composed of InGaAsP based semiconductor materials, theelectron speed (transit time) in the first semiconductor opticalabsorption layer is sufficiently higher (shorter) than the electronspeed (transit time) in the second semiconductor optical absorptionlayer. Accordingly, the design range of the optical absorption layer isactually present, where an approximation for R₂(ω) given by R₂₂(ω) ispossible.

On the other hand, almost all the current caused by the electrons andholes generated in the first semiconductor optical absorption layerflows toward an external circuit because both ends of the layer are in acharge neutral state (majority carrier is present). Thus, the conditionR_(tot)(ω) to R₁(ω)+R₂(ω) can be achieved, which means that the responseis obtained just by superimposing the responses in parallel in thestructure in which the first semiconductor optical absorption layer andthe second semiconductor optical absorption layer are presentindependently.

Here, a total response J(ω) of the circuit through which the diffusioncurrent and drift current flow in parallel in the charge control modelis given by the following expression,

 J(ω)=J _(DC)[(W _(A) /W)/(1+jωτ _(A))+(W _(D) /W)/(1+jωτ _(D))].  (13)

It is approximated by the following expression in the low frequencyregion,J(ω)≈J _(DC)[1−jω(W _(A)τ_(A) +W _(D)τ_(D))/W].  (14)Consequently, the total carrier transit time τ_(tot) is given by thefollowing expression, $\begin{matrix}\begin{matrix}{\tau_{tot} \approx {( {{W_{A}\tau_{A}} + {W_{D}\tau_{D}}} )/W}} \\{= {{\lbrack {{W_{A}^{2}/( {3D_{e}} )} + {W_{A}/v_{th}}} \rbrack( {W_{A}/W} )} +}} \\{\lbrack {{W_{D}/3}V_{h}} \rbrack( {W_{D}/W} )} \\{= {{W_{A}^{3}/( {3W\quad D_{e}} )} + {W_{A}^{2}/( {W\quad v_{th}} )} + {W_{D}^{2}/{( {3W\quad v_{h}} ).}}}}\end{matrix} & (15)\end{matrix}$

To improve the response speed, it is necessary to minimize τ_(tot) givenby the foregoing expression (15). Accordingly, the response speed (andits measure 3 dB bandwidth) can be maximized by setting the ratiobetween W_(A) and W_(D).

As shown in the expression (15), τ_(tot) consists of the components[W_(A) ²/(3D_(e))+W_(A)/v_(th)](W_(A)/W) and [W_(D)/3v_(h)](W_(D)/W).Thus, it depends on the ratios of W_(A) and W_(D) to the total opticalabsorption layer width W. In this case, the carrier transit times of thephotodiodes with the structures where W_(A)=0 (only the depletedsemiconductor optical absorption layer) and W_(A)=W (only the p-typeneutral semiconductor optical absorption layer) are equal to the carriertransit times given by the foregoing expressions (1) and (3).

In contrast with this, the τ_(tot) of the photodiode in accordance withthe present invention, in which both W_(A) and W_(D) have a finite filmthickness can be smaller than W²/(3D_(e))+W/v_(th) and W/3v_(h).Accordingly, it can achieve τ_(tot) smaller than that of the pin-PD andUTC-PD, thereby enabling a faster response.

As described above, the photodiode in accordance with the presentinvention with the structure as shown in FIGS. 1 and 3 can reduce themutually dependent relationship between the two semiconductor opticalabsorption regions. As a result, the photodiode has the total responseobtained by superimposing the responses of the structures in parallel inwhich these two regions are present independently.

FIG. 4 is a graph illustrating the calculation results of the totalcarrier transit time τ_(tot) and the 3 dB bandwidth f_(3dB), which areobtained on a basis of the foregoing model in the optical absorptionlayer of the photodiode in accordance with the present inventionincluding the depleted InGaAs optical absorption layer with the layerwidth W_(D) and the p-type neutral InGaAs optical absorption layer withthe layer width W_(A). It illustrates the variations in the totalcarrier transit time τ_(tot) and the 3 dB bandwidth f_(3dB) for theratio of W_(A) to W under the conditions that the diffusion coefficientof the electrons is D_(e)=200 cm²/s, the hole speed is v_(h)=5×10⁶ cm/s,and the total thickness of the InGaAs optical absorption layer isconstant at W=W_(A)+W_(D)=0.4 μm.

As is seen from FIG. 4, the photodiode in accordance with the presentinvention has the maximum value 116 GHz for f_(3dB) at W_(A)=0.18 μm(W_(D)=0.22 μm). In contrast, the pin-PD has f_(3dB) of 60 GHz atW_(A)=0 (W_(D)=W), and the UTC-PD has f_(3dB) of 37 GHz at W_(A)=W(W_(D)=0). Thus, the photodiode in accordance with the present inventioncan increase f_(3dB) sharply. Since the τ_(tot) increases with anincrease in the total optical absorption layer width W, the combinationof W_(A) and W_(D) that maximizes the bandwidth is also the combinationof maximizing the internal quantum efficiency for a given bandwidth.

(EMBODIMENT 2)

FIG. 5 is a cross-sectional view showing a structure of the photodiodein accordance with the present invention. The photodiode includes ann-type semiconductor electrode layer 51 composed of a single-layer ormultiple-layers with different band gaps; a semiconductor opticaltransmission layer 52 depleted in the reverse-biased state; a firstsemiconductor optical absorption layer 53 with a film thickness W_(D)which is depleted in the reverse-biased state; a second semiconductoroptical absorption layer 54 with a film thickness W_(A) which is p-typeneutral in the reverse-biased state; and a p-type semiconductorelectrode layer 55 composed of the single-layer or multiple-layers withdifferent band gaps. The photodiode is compose of these layers stackedsequentially, and an n-type electrode 56 formed on the n-typesemiconductor electrode layer 51, and a p-type electrode 57 formed onthe p-type semiconductor electrode layer 55. The semiconductor opticaltransmission layer 52 is designed to have a greater band gap than thefirst semiconductor optical absorption layer 53.

FIG. 6 is a band diagram illustrating a drift of carriers generated inthe p-type neutral semiconductor optical absorption layer and thedepleted semiconductor optical absorption layer as shown in FIG. 5. Whenthe semiconductor optical absorption layer of a semiconductor structureis illuminated which includes an n-type semiconductor electrode layer 61composed of a single-layer or multiple-layers with different band gaps,a semiconductor optical transmission layer 62 depleted in thereverse-biased state, a first semiconductor optical absorption layer 63with a film thickness of W_(D) which is depleted in the reverse-biasedstate, a second semiconductor optical absorption layer 64 with a filmthickness of W_(A) which is p-type neutral in the reverse-biased state,and a p-type semiconductor electrode layer 65 composed of thesingle-layer or multiple-layers with different band gaps, which arestacked sequentially, electron-hole pairs are generated in theindividual semiconductor optical absorption layers. The electrons drifttoward the n-type semiconductor electrode 61, whereas the holes drifttoward the p-type semiconductor electrode 65, thereby implementing thecondition in which the current flows through a parallel circuit, so tospeak.

In the photodiode with the structure as shown in FIG. 5, thesemiconductor optical transmission layer 52 is designed to be depletedin the reverse-biased state. Accordingly, a marked increase does notoccur of the electron charges in the semiconductor optical transmissionlayer 52 during the operation, thereby being able to increase the pnjunction width and to reduce the junction capacitance. In addition, thecarrier transit time through the depleted first semiconductor opticalabsorption layer 53 little varies as long as the transit time of theholes is constant, and hence the total carrier transit time τ_(tot)little increases.

The total carrier transit time (τ_(tot)) is given by the followingexpression by handling the diode response according to the chargecontrol model as in the foregoing embodiment 1 under the conditions thatthe first semiconductor optical absorption layer has a layer widthW_(D), the second semiconductor optical absorption layer has a layerwidth W_(A), the semiconductor optical transmission layer has a layerwidth W_(T), and that the sum W (=W_(D)+W_(A)) of the layer widths ofthe first and second semiconductor optical absorption layers isconstant,τ_(tot) =W _(A) ³/(3WD _(e))+W ² _(A)/(Wv _(th))+[W _(D) ³/(W _(D) +W_(T))]/(3Wv _(h)).  (16)Comparing the result with the foregoing expression (15), it is seen thatthe third term of the expression (16) is reduced by an amountW_(D)/(W_(D)+W_(T)).

In other words, the photodiode with the structure as shown in FIG. 5 hasan advantage that even when W_(A) and W_(D) are reduced, the pn junctionwidth is not narrowed, and hence the bandwidth is not impaired with anincrease of a junction capacitance C and RC time constant, thereby beingable to implement a photodiode with a structure advantageous to thehigh-speed operation.

INDUSTRIAL APPLICABILITY

As described above, the photodiode in accordance with the presentinvention has the optical absorption layer composed of the depletedfirst semiconductor optical absorption layer with the layer width W_(D)and the p-type neutral second semiconductor optical absorption layerwith the layer width W_(A), and sets the ratio between W_(A) and W_(D)such that the total carrier transit time τ_(tot) becomes minimum. As aresult, it can greatly alleviate the problem of the “tradeoff betweenthe response speed and internal quantum efficiency” of the conventionalphotodiode.

In particular, the photodiode in accordance with the present inventioncan obviate the restriction on the design of the conventional photodiodeused for a ultrafast signal processing (≧40 Gbit/s) that it must securethe bandwidth at the expense of the internal quantum efficiency byreducing the optical absorption layer width.

1. A photodiode including an n-type semiconductor electrode layer, anoptical absorption layer having a first semiconductor optical absorptionlayer and a second semiconductor optical absorption layer having ap-type, and a p-type semiconductor electrode layer, which are stackedsequentially, said photodiode further including an n-type electrodeformed on said n-type semiconductor electrode layer, and a p-typeelectrode formed on said p-type semiconductor electrode layer, whereinwhen a specified reverse-bias is applied across said n-type electrodeand said p-type electrode, said first semiconductor optical absorptionlayer is depleted, and said second semiconductor optical absorptionlayer remains neutral in a region other than a region near an interfacewith said first semiconductor optical absorption layer, and wherein insaid optical absorption layer with a fixed layer width equal to a sum(W=W_(D)+W_(A)) of a layer width (W_(D)) of said first semiconductoroptical absorption layer and a layer width (W_(A)) of said secondsemiconductor optical absorption layer, a ratio between W_(D) and W_(A)is determined such that |Δτ_(A)|=|−Δτ_(D)| holds, where −Δτ_(D) is adecrease of a hole transit time through said first semiconductor opticalabsorption layer and Δτ_(A) is an increase of an electron transit timethrough said second semiconductor optical absorption layer, when thelayer width of said first semiconductor optical absorption layer is madeW_(D)−ΔW, and the layer width of said second semiconductor opticalabsorption layer is made W_(A)+ΔW.
 2. The photodiode as claimed in claim1, further including a semiconductor optical transmission layer betweensaid first semiconductor optical absorption layer and said n-typesemiconductor electrode layer, said semiconductor optical transmissionlayer having a band gap greater than said first semiconductor opticalabsorption layer, wherein said semiconductor optical transmission layeris depleted when a specified reverse-bias is applied across said n-typeelectrode and said p-type electrode.
 3. The photodiode as claimed inclaim 1, wherein said first semiconductor optical absorption layer andsaid second semiconductor optical absorption layer are composed of asame or different compound semiconductor with composition represented byIn_(x)Ga_(1-x)As_(y)P_(1-y), where 0≦x and y≦1.
 4. The photodiode asclaimed in claim 3, further including a semiconductor opticaltransmission layer between said first semiconductor optical absorptionlayer and said n-type semiconductor electrode layer, said semiconductoroptical transmission layer having a band gap greater than said firstsemiconductor optical absorption layer, wherein said semiconductoroptical transmission layer is depleted when a specified reverse-bias isapplied across said n-type electrode and said p-type electrode.
 5. Thephotodiode as claimed in claim 3, wherein the ratio between said W_(D)and W_(A) is set such that τ_(tot) given by the following expressionbecomes minimum,τ_(tot) =W _(A) ³/(3WD _(e))+W _(A) ²/(Wv _(th))+W _(D) ²/(3Wv _(h)),where D_(e) is a diffusion coefficient of electrons, v_(th) is athermionic emission velocity of the electrons, and v_(h) is a driftspeed of holes.
 6. The photodiode as claimed in claim 5, furtherincluding a semiconductor optical transmission layer between said firstsemiconductor optical absorption layer and said n-type semiconductorelectrode layer, said semiconductor optical transmission layer having aband gap greater than said first semiconductor optical absorption layer,wherein said semiconductor optical transmission layer is depleted when aspecified reverse-bias is applied across said n-type electrode and saidp-type electrode.